Among mobile communication systems such as portable telephone systems, cellular-based communication systems are mainstream. A cellular-based communication system combines multiple areas (cells) respectively of a communicable range of a base station to cover a wide area, and while switching communicable base stations according to the movement of a mobile station, maintains communication. Currently, although third generation mobile communication services are provided by code division multiple access (CDMA), a subsequent generation mobile communication system is under consideration that enables communication at higher speeds.
For example, long-term evolution (LTE) and LTE-advanced are under investigation in the 3rd Generation Partnership Project (3GPP). Under the LTE and LTE-advanced schemes, to improve the efficiency of frequency utilization, an operation method is assumed where communication is executed in each cell using the same frequency and a mechanism is introduced to control inter-cell interference, which is a problem associated with the operation method.
For example, in an environment where multiple cells employing different levels of transmission power such as a macro cell, a pico cell, and a femto cell (a heterogeneous network), mutual interference becomes heavy when these cells are operated using the same frequency. In particular, a mobile station located near the border of the cells is subject to heavy interference caused by a neighboring cell. Therefore, communication quality is significantly degraded.
As for the downlink, under LTE Rel. 8, a method is proposed where among the cells, notification of the maximum transmission power for each frequency is given, whereby transmission power distribution that reduces inter-cell interference is determined for the cells. By this method, inter-cell interference is reduced for data communication.
For example, under LTE, a control signal and a data signal are time-division multiplexed in each transmission slot. In a system whose cells are synchronized, the slot transmission timing of each of the cells is same and therefore, inter-cell interference occurs among control-signal multiplexed sections and among data-signal multiplexed sections. The inter-cell interference of the data signal portion can be alleviated by, for example, using a method provided in LTE Rel. 8. However, the inter-cell interference of the control signal portion cannot be reduced even by using this method.
Under 3GPP, a method has been proposed of preventing interference among the control signals by shifting sub frames of a macro cell and a pico cell by the length of the control signal section (see, e.g., “Interference Coordination for Non-CA-based Heterogeneous Networks”, 3GPP, R1-102307). In “Interference Coordination for Non-CA-based Heterogeneous Networks”, 3GPP, R1-102307, a method is proposed of setting a new control channel region for a pico cell in a conventional data channel region.
However, in the conventional techniques, a problem arises in that the throughput of a cell subject to interference deteriorates. For example, interference occurs between a control signal and a data signal with the method of preventing the interference among the control signals by shifting sub frames of a macro cell and a pico cell by the length of the control signal section and therefore, the throughput of the macro cell deteriorates. The interference with a pico cell control signal cannot be reduced in a sub frame for the macro cell to transmit the data signal.
With the method of setting a new control channel region for a pico cell in a conventional data channel region, the throughput of the data deteriorates because the region that can multiplex the data channel is reduced. Because a new channel is defined, another problem also arises in that the backward compatibility is lost.